While the opener proved to be a sprint, the second 'Red Group' tie was a marathon when Maria Sharapova beat Agnieszka Radwanska 4-6 6-3 6-4 in a full-blooded encounter marked by several swings in momentum as both players struggled on serve.
World number two Halep is the highest ranked player in the field after Serena Williams, who has won the tournament for the last three years, withdrew citing the need to rest after a year spent carrying injuries in her quest for a calendar Grand Slam.
Halep crushed Williams in round robin play a year ago before losing to the American in the final and her love of the Singapore Indoor Stadium hard court was evident in a first set she wrapped up in 26 minutes after breaking serve three times.
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"I feel the same now as I did against Serena," Halep told reporters of the win. "I can play my best tennis here yet I didn't play my best today, but I played good tennis.
"I played solid and was dominating the match," she added.
The Romanian had arrived in Singapore under an injury cloud after picking up an ankle knock at the start of October, but the 24-year-old moved effortlessly around the court on Sunday and withstood Pennetta's attempts to get back into the contest.
The 33-year-old Italian played more like she had on her way to a stunning victory in New York last month in the second set but her resistance ended in a marathon eighth game when she dropped serve to trail 5-3 after staving off three break points.
HIGH INTENSITY
Pennetta, who announced she would retire from tennis at the end of the season, forced only two break points, both in the opening game of the second set, during the entire match and her unforced error count of 27 made life much easier for Halep.
Sidelined by leg and arm injuries, Sharapova had yet to complete a match since Wimbledon but after shaking off some initial rust, the Russian was back to her very best against Radwanska, playing each point as if her life depended on it.
A single break was enough to give the Pole an edge in the opener but her excellent retrieving skills could do little to prevent the world number three from rallying for victory once the Russian had raised her intensity levels through the roof.
"Coming into this event, I tried to do everything I could to be healthy and although I didn't play my best tennis today, sometimes that's not what matters," Sharapova said of a match that lasted two hours, 48 minutes with a large portion of games going to deuce.
"I'm so happy that I got through and have a chance to play two more matches to end the season and hopefully more, so I couldn't be happier at this point."
The 'White Group' consists of Garbine Muguruza, Petra Kvitova, Angelique Kerber and Lucie Safarova, with play in that pool starting on Monday.
(Editing by Peter Rutherford and Toby Davis)
